The invention relates to a coating device for coating a travelling web of material with a coating liquid comprising means for transporting the web along a path of travel and a coating hopper positioned above the path of travel and which extends substantially at right angles above it and is provided with a pouring lip, in particular a slide hopper with at least one slide surface for producing a liquid curtain which falls freely onto the web to be coated.
Cascade coating is one of the most efficient precision coating methods and is used in particular for producing photographic materials. Cascade coating methods and slide hoppers suitable therefor are described in more detail, inter alia, in U.S. Pat. Nos. 3,508,947 and 3,632,347.
Of the numerous known cascade coaters, two kinds chiefly are of particular importance, viz. slot hoppers and slide hoppers. In the slot hoppers, the coating liquid issues at the lower end of a metering slot situated substantially at right angles above the material to be coated and directly forms a free-falling curtain. In the slide hoppers on the other hand, the coating liquid is forced through a metering slot exiting into a downwardly inclined slide surface, then flows in a thin film down the slide surface under the force of gravity and only forms the free-falling curtain at the lower end of the slide surface when leaving this latter. In both hoppers, the liquid curtain can have one or more layers.
It will be appreciated that the prerequisite for a precise, even coating over the entire width of the surface to be coated is a correspondingly even thickness of the free-falling curtain. To satisfy this prerequisite, two essential requirements must be met: the exit or metering slot, which determines the thickness of the layer of liquid, must be so precisely constructed and dimensioned that the rate of flow of the liquid over the entire width to be coated is uniform.
The lip, i.e. the point where the free-falling curtain forms from the layer of liquid bounded on one or two sides, must be so formed that the passage of the flow of liquid is free from disturbance, in particular from lateral speed components.
The first requirement can be satisfied without any great difficulties by applying knowledge of the laws of hydrodynamics and by as precise a construction as possible of the hopper. In practice, however, considerable difficulties are encountered in satisfying the second requirement. For it becomes apparent that, during the transition from the flow of a layer bounded on one or two sides to the free unrestricted fall, disturbances which have their origin in a nonuniform, uneven wetting line can arise. Such wetting lines are necessarily formed wherever the liquid leaves the boundary wall on one or both sides and begins to form a free boundary surface with the atmosphere. If as a consequence of the geometrical formation of the pouring zone, i.e. the location where the free-falling curtain forms, the wetting line falls on a substantially even or relatively slightly curved surface, then in virtually every case an irregular, uneven wetting line is formed. The cause thereof lies in the impossibility of forming the surface of a solid material so evenly that a uniform wetting is ensured. Slight differences in roughness or traces of impurities result in locally uneven angles of wetting and thus in an irregularly curved wetting line. The unavoidable consequence thereof is an uneven initial thickness of the curtain, which, because of the strictly laminary flow, is transmitted over the entire curtain to the coating and appears there in the form of thicker or thinner longitudinal zones.